chore: consolidate parsing logic

Author: bchampp

src/aws_durable_execution_sdk_python_testing/web/handlers.py

@@ -198,10 +198,7 @@ def _parse_callback_result_payload(self, request: HTTPRequest) -> bytes:
         Raises:
             InvalidParameterValueException: If payload parsing fails
         """
-        if not isinstance(request.body, bytes):
-            return b""
-
-        if not request.body:
+        if not request.body or not isinstance(request.body, bytes):
             return b""
 
         try: